According to conventional bevel polishing systems for polishing a bevel portion of a substrate such as a wafer, a pad called a polishing or pressing pad is pressed against the bevel portion of the substrate through a polishing tape with a proper load. The systems thus control the amount of polishing of the substrate and the shape of the substrate. The polishing pad is controlled by means of, for example, pneumatic control using an electropneumatic regulator. The electropneumatic regulator is used to adjust an air pressure supplied to an air cylinder to desired pressure, to thereby control the pressing force of the polishing pad and thus control the pressure which presses the polishing surface of the polishing tape against the substrate (Patent Document 1).
Thus, as described above, the bevel polishing systems use the electropneumatic regulator for precise control on air pressure in the air cylinder so as to maintain polishing performance. It is therefore required to obtain an accurate correlation between the air pressure in the air cylinder and the pressing load at an end of the polishing pad. As a method for fulfilling this requirement, calibration of a load from the polishing pad is currently performed by actually measuring the pressing load with a force gauge under controlled air pressure conditions.